1. Field of the Invention
The invention relates to transparent toilet soaps of improved color, a method for achieving color reduction in such soaps, and a new reducing agent system.
2. The Prior Art
Commercially available transparent toilet soaps tend to be rather darkly colored. This color may be an inherent property of unsaturated fatty acid soap and so be intrinsic to the raw materials. Alternatively, the color may arise from reactions during processing.
Formulations subject to discoloration induced by processing, in particular heat sensitive formulations, are those which contain alkanolamines and/or alkanolamine salts. It is during heating that the alkanolamines and their salts oxidize to form minute quantities of highly colored compounds. The resulting soap bar will, therefore, display a characteristic brown hue. Many consumers find brown to be an aesthetically unappealing toilet bar color.
Reducing agents might be expected to inhibit discoloration by reacting with the chromophores of color generating bodies. Indeed, the patent literature records a number of transparent soap formulations with reducing agents.
U.S. Pat. Nos. 3,926,828 and U.S. 3,793,214 to O'Neill et al. disclose the use of sodium hydrosulfite in a transparent soap at concentrations ranging from 0.01 to 0.05 wt.%. U.S. Pat. No. 4,207,198 to Kenkare teaches that sodium bisulfite may be added at a concentration of 0.5 wt.% as chemical stabilizer to squeezable, elastic detergent bars which may or may not be transparent. These detergent bars are substantially anhydrous and consist essentially of gelatin and synthetic detergents. U.S. Pat. No. 4,468,338 to Lindberg reports that alkali metal sulfite, bisulfite and metabisulfite can be used as discoloration preventing additives in transparent soap at concentrations ranging from 0.2 to 1.0 wt.%. These sulfur additives are effective only if citric acid and/or related compounds are also present. Japanese Pat. No. 59-6300 (Shiseido) reports transparent soaps blended with 0.05 to 1.0 wt.% sodium sulphide providing a medical benefit against acne. Pleasant pale yellow or brown colors are said to be characteristics of this soap. Finally, German Patents DE No. 1,938,177 and DE No. 1,938,178 to Henkel discloses lightly colored fatty acid soaps containing either hydrazine, hydroxylamine or alkali metal salts of 2 and 4 valent sulphoxo acids, e.g. sodium sulfite, as reducing agents in amounts preferably from about 0.01 to 5 wt.%.
One of the problems with known reducing agents is that these compounds have a finite solubility in soap systems. When this solubility is exceeded, the reducing agent will crystallize out as solid crystals thereby adversely affecting transparency. Moreover, it is known that electrolytes reduce the solubility of soaps in water. Thus, where the reducing agent is also an electrolyte, the soap itself would have an increased tendency to crystallize out as solid crystals further adversely affecting transparency. Accordingly, it would be desirable to find reducing systems operative at lower concentration levels than disclosed in the known art. Smaller amounts of reducing agent will, in turn, permit improved transparency.
It is, therefore, an object of the present invention to provide a color reducing system for toilet soap bars effective at lower electrolyte level than previously known.
It is another object of the present invention to substantially reduce the color while improving the transparency of presently known transparent soap bars.
Furthermore, it is an object of this invention to provide a method for inhibiting discoloration of soap bars in general and provide an improved reducing system.